This invention relates to a reflective mask blank having a programmed defect (or artificial defect) and a reflective mask having a programmed defect for use in inspection or evaluation of a reflective mask for exposure for use in pattern transfer in a semiconductor process and a reflective mask blank for use in producing the reflective mask. This invention also relates to a method of producing the reflective mask blank having a programmed defect and a method of producing the reflective mask having a programmed defect.
In the semiconductor industry, reduction in size of a semiconductor device is more and more accelerated in recent years. Following the reduction in size of the semiconductor device, an exposure technique capable of achieving higher resolution is required. As the exposure technique of the type, attention is directed to an EUV (Extreme Ultra Violet) lithography using EUV light. It is noted here that the EUV light is a radiation having a wavelength within a soft X-ray region or a vacuum ultraviolet region, specifically, within a range between about 0.2 and 100 nm. As a mask used in the EUV lithography, reflective masks for exposure are proposed in Japanese Patent Application Publications (JP-A) Nos. H08-213303 and H07-333829.
The reflective mask of the type comprises a substrate, a reflective multilayer film formed on the substrate to reflect light, a buffer layer formed on the reflective multilayer film, and an absorber film formed on the buffer layer in a predetermined pattern to absorb light. In an exposure apparatus, light incident to the reflective mask is partially absorbed in an area where the absorber film is present and is partially reflected by the reflective multilayer film in a remaining area where the absorber film is not present. The former area and the latter area may be referred to as an absorbing region and a reflecting region, respectively. An image formed by the light reflected by the reflective multilayer film is transferred onto a wafer through a reflection optical system. Herein, the buffer layer serves to protect the reflective multilayer film when the predetermined pattern of the absorber film is formed by dry etching or the like in a mask production process.
Generally, evaluation of a transferability of a mask used in pattern transfer in a semiconductor process and inspection of a defect of the mask are carried out by the use of a mask or a mask blank provided with a programmed defect formed at a predetermined position. For example, in order to carry out a transferability test or a defect inspection for a white defect (a part where the absorber film is lost) or a black defect (a part where the absorber film is excessive) formed on the mask, preparation is made of a mask having a programmed defect or a mask blank having a programmed defect in which a programmed defect, i.e., a white defect (a clear defect) or a black defect (an opaque defect), having a predetermined size is preliminarily formed at a predetermined position. The mask or the mask blank having a programmed defect is evaluated for the transferability or used as a reference sample in the inspection to calibrate the sensitivity of an inspection apparatus and to quantitatively evaluate a defect in a product.
If the above-mentioned reflective mask has an irregularity formed on a mask surface in the vicinity of the pattern, the reflected light will be changed in phase due to the irregularity. The change in phase results in degradation in positional accuracy and contrast of a transferred pattern. A defect causing the change in phase affecting the transferability may be called a phase defect.
In case of the existing photolithography, exposure light has a relatively long wavelength within an ultraviolet region (between 150 and 247 nm). Therefore, even if the irregularity is produced on the mask surface, the phase defect is hardly caused. Such irregularity produced under an ordinary manufacturing condition results in no serious problem. Further, even if the irregularity is formed, a wide tolerance is assured. Therefore, it is not essential to inspect or evaluate the phase defect.
However, in case where short-wavelength light, such as the EUV light, is used as the exposure light, the change in phase is very sensitive with respect to a microscopic irregularity on the mask surface, resulting an increase in influence upon a transferred image. Therefore, the change in phase resulting from such a very small irregularity is not negligible. For example, in case where the EUV light having a wavelength of about 13 nm, even the irregularity of about 2 nm may become the phase defect.
Therefore, in the lithography in a short-wavelength region, it is essential to research or evaluate the influence of the phase defect in the mask upon the transferred pattern and to quantitatively inspect the phase defect in production of the mask or the mask blank.
In order to evaluate or inspect the phase defect in the above-mentioned reflective mask, it is necessary to prepare a mask having a programmed defect or a mask blank having a programmed defect in which a phase defect of a predetermined size is preliminarily formed. However, such a reflective mask blank having a programmed defect or such a reflective mask having a programmed defect for use in inspection and evaluation is not known so far. Further, as the phase defect, a microscopic irregularity on the order of several nanometers must be formed. However, any technique for forming such a microscopic phase defect on the reflective multilayer film at a particular position with a particular size is not known.
In defect inspection, the reflective multilayer film must have a flat and smooth film surface in view of the sensitivity of a defect inspection apparatus. At present, such a mask blank having a programmed defect in which the reflective multilayer film has a flat and smooth surface and a method of producing the same are not known.